At least 12 workers were killed after a portion of an under-construction tunnel collapsed following a gas explosion in India's north-eastern Sikkim state, officials said.

Rescue operations are continuing to locate 13 others who are reported to be trapped in the tunnel since Monday's collapse.

The National Disaster Response Force (NDRF) said the incident happened about 1.5km (0.9 miles) inside the tunnel.

The under-construction tunnel, designed to carry water from the nearby Teesta River to a hydropower plant's turbines, is part of a larger project in the Himalayan state being developed by the state-owned National Hydroelectric Power Corporation (NHPC).

The accident happened "after [a] sudden burst of suspected methane gas trapped/embedded inside the rocks which led to [an] explosion generating dense fumes and toxic gases", NHPC said in a statement.

In a separate statement, the Sikkim government said the accident was caused by "methane gas combustion".

Methane is a naturally occurring gas that can sometimes be trapped underground. If tunnelling releases it into a confined space and it comes into contact with a spark or another ignition source, it can ignite or explode, a government official said.

There were 27 people working in different sections inside the tunnel - of which two managed to escape after the explosion and 25 were trapped.

"Of the 25 people who were trapped, we have recovered 12 bodies so far, and teams are working to locate the 13 other men," Superintendent of Police Sonam D Bhutia told the BBC on Wednesday morning.

The police say they have identified seven victims. One of them is from Sikkim, while others are migrants from West Bengal, Uttarakhand and Assam states.

Officials say their cause of death will be ascertained after a post-mortem examination.

The ongoing search-and-rescue operation is focused on a specific 200-metre zone where the remaining people are likely to be located, NDRF officer Mohsen Shahedi told ANI news agency.

Chief Minister Prem Singh Tamang has set up a special investigation team in connection with the incident.

"We have engaged a team of experts to find out what exactly happened and how. We will take action on the basis of the findings," Tamang told reporters. (BBC)